Stressed Fin-FET Devices with Low Contact Resistance

ABSTRACT

A method for fabricating an FET device is disclosed. The method includes Fin-FET devices with fins that are composed of a first material, and then merged together by epitaxial deposition of a second material. The fins are vertically recesses using a selective etch. A continuous silicide layer is formed over the increased surface areas of the first material and the second material, leading to smaller resistance. A stress liner overlaying the FET device is afterwards deposited. An FET device is also disclosed, which FET device includes a plurality of Fin-FET devices, the fins of which are composed of a first material. The FET device includes a second material, which is epitaxially merging the fins. The fins are vertically recessed relative to an upper surface of the second material. The FET device furthermore includes a continuous silicide layer formed over the fins and over the second material, and a stress liner covering the device.

BACKGROUND

The present invention relates to electronic devices of very large scaleintegration (VLSI) circuits. In particular, it relates to Fin type FETdevices.

As FET (Field-Effect-Transistor) devices are being scaled down, thetechnology becomes more complex, and changes in device structures andnew fabrication methods are needed to maintain the expected performanceimprovements from one successive device generation to the next. Fin-FETor Tri-Gate devices offer possible avenues to continue on the path ofminiaturization.

BRIEF SUMMARY

A method for fabricating an FET device is disclosed. The method includesreceiving Fin-FET devices with fins that are composed of a firstmaterial. The method also includes the merging of the fins by anepitaxial deposition of a second material, and recessing the finsvertically using a selective etch. Furthermore, a continuous silicidelayer is formed over the first material and over the second material,followed by deposition of a stress liner overlaying the silicide.

An FET device is disclosed, which FET device includes a plurality ofFin-FET devices, the fins of which are composed of a first material. TheFET device includes a second material, which is epitaxially merging thefins to one another in pairwise manner. The fins are recessed in avertically offset position relative to an upper surface of the secondmaterial. The FET device furthermore includes a continuous silicidelayer formed over the fins and over the second material, and a stressliner overlaying the silicide layer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

These and other features of the present invention will become apparentfrom the accompanying detailed description and drawings, wherein:

FIGS. 1A and 1B symbolically show respective inclusive, and crosssectional views of an initial stage for an embodiment of the disclosure;

FIGS. 2A and 2B symbolically show respective inclusive, and crosssectional views after an epitaxial merging of the fins;

FIGS. 3A and 3B symbolically show respective inclusive, and crosssectional views after the recessing of the fins;

FIGS. 4A and 4B symbolically show respective inclusive, and crosssectional views after silicidation of the fins and of the mergingmaterial; and

FIG. 5 symbolically shows a cross sectional view after deposition of astress liner.

DETAILED DESCRIPTION

It is understood that Field Effect Transistor-s (FET) are well known inthe electronic arts. Standard components of an FET are the source, thedrain, the body in-between the source and the drain, and the gate. Thegate is overlaying the body and is capable to induce a conductingchannel in the body between the source and the drain. In the usualnomenclature, the channel is hosted by the body. The gate is typicallyseparated from the body by the gate insulator, or gate dielectric.Depending whether the “on state” current in the channel is carried byelectrons or by holes, the FET comes in two types: as NFET or PFET. (Indifferent nomenclature the NFET and PFET devices are often referred toas NMOS and PMOS devices.)

A Fin-FET, or Tri-Gate, device is an FET device but with a particulargeometric configuration. These devices are non-planar, they are threedimensional structures hosted by a fin structure. In Fin-FETs, the bodyof the transistor is formed in a fin rising out of a planar background,typically having both vertical and horizontal surfaces. The gate of theFin-FET may engage the top surface, as well as the vertically orientedbody surfaces on both faces, or sidewalls, resulting in several planesbeing used for transistor channel formation. Such FET devices, withfin-type bodies, have several advantages as known in the art. Inembodiments of the present invention, the dimensions of fin structuresthat serve as fin-type device bodies may be of a height of about 5 nm to50 nm, and of a width of between 3 nm to about 30 nm.

There is great difficulty in maintaining performance improvements indevices of deeply submicron generations. One general approach forimproving performance is to try to increase carrier (electron and/orhole) mobilities. A promising avenue toward better carrier mobility isto apply tensile or compressive stress in the semiconductor channelregions. Typically, it may be preferable to have the channel of electronconduction type devices, such as NFET, in tensile stress, while to havethe channel of hole conduction type devices, such as PFET, incompressive stress. In the case of the more common planar devices amethod know in the art for stressing channels is the deposition of socalled stress liners. Stress liners are insulating layers covering thedevices, which layer are under stress, and then impart the stress ontothe device channels. It is known that the stress distribution is suchthat if a stressing layer which covers a planar FET, is, for instance,in tensile stress, then the channel of the FET will also be undertensile stress, while the source and drain of the same device would beunder compressive stress. For compressive stress one has the mirrorsituation: a stress liner in a compressive state of stress impartscompressive stress onto the device channel. Consequently, one maygenerally apply tensile stressing layers to cover NFET devices andcompressive stressing layers to cover PFET devices.

Applying stress to non-planar, three dimensional (3D) FETs, such as aFin-FET, or Tri-Gate device, may be more difficult. Stress linerscommonly used for planar devices are not very efficient, partly becauseof the 3D nature of the device, and partly because of the shrinking ofthe device pitch as technology progresses. Smaller device pitch leads tothinner stress liners, and that results in less channel stress. One wayto increase the stress coupling to Fin-FET channel regions is to recess,namely etch down, the source/drain area of the fins. However, therecessed fins may lead to a significant increase in the device parasiticresistance. An embodiment of the present invention increases stresscoupling to the channel and may not effect negatively the deviceparasitic resistance.

FIG. 1A symbolically shows an inclusive view of an initial stage of anembodiment of the disclosure. This initial stage is that of receiving aplurality of Fin-FET devices. The term of receiving is intended to beinclusive of any possible manner by which one may arrive at this initialstage of an embodiment. The plurality of Fin-FET devices may have justreached this stage of fabrication, or they may have been supplied by ansome source for the purpose to apply the embodiments of the instantinvention. FIG. 1A shows three Fin-FET devices, however this is only forillustrative purposes. Embodiments of the present disclosure may haveonly two Fin-FET devices, or may have several hundreds, or more. TheFin-FET device arrangement shown in FIG. 1A usually is configured to actas a single FET device. Often in a given integrated circuit (IC) aFin-FET device is manufactured having only a single device width, whichwidth is that portion of the fin circumference that is engaged by thegate. Multiple device widths for circuit operations are achieved byconnecting single Fin-FET devices to each other in parallel. This may beaccomplished by electrically connecting the source side fins to eachother, and the drain side fins to each other.

Manufacturing of NFET, PFET, and Fin-FET devices is established in theart. It is understood that there are large number of steps involved insuch processing, and each step may have practically endless variations,known to those skilled in the art. For embodiments of this disclosure itis understood that the whole range of known processing techniques areavailable for fabricating the devices, and only those process steps willbe detailed that are related to the embodiments of the presentinvention.

The Fin-FET devices, as displayed in FIG. 1A, as well as all Fin-FETdevices in embodiments of the present disclosure may be any kind ofFin-FET devices, fabricated in any manner. They may be, for example, andwithout intent of limiting, oxide dielectric or high-k dielectricdevices, they may be polysilicon gate or metal gate devices, or of anyfurther kind. The fabrication may have so called gate first, or gatelast, variations, or further kinds The semiconductor material of thefins may be the mainstay material of microelectronics, namely silicon(Si). But, more broadly, they may be of Si based materials, such as asilicon-germanium (SiGe) alloy, or Si with a carbon content Si:C.Furthermore, other semiconductors, such as compound semiconductors, arenot excluded. The fin material typically is in single-crystal form.

Accordingly, the Fin-FET devices shown in the figures may be regarded asrepresentative embodiments of Fin-FET devices in general. In typicalembodiments of the instant invention the Fin-FET devices are fabricatedover a platform 10, which maybe be a buried oxide layer (BOX). Such BOXlayer is typical in the art, but embodiments of the present invention donot depend on the platform 10 being a BOX layer. The platform 10supporting the Fin-FET devices could be of other insulators,semiconductors, or metallic substances, or any other is use withoutlimitation.

In embodiments of the present disclosure, the material that the fins arecomposed of is referred to as the first material. The first material 20of the fins rests over the platform 10. On one side of the gate 60 thefins are the source terminals, on the other side of the gate the finsare the drain terminals. FIG. 1A also shows spacers 55, and there may befurther sub-layers involved in, and around, the gate 60. As an example,a hardmask 65, which may have been used in patterning and processing thegate 60 and the spacers 55, is also shown. This mask layer 65 may or maynot be removed during further processing.

The dashed line, delimited with “x”-s in FIG. 1A, indicates a cut linewhere all the cross sectional views of the figures of the disclosure areshown, including FIG. 1B. The direction of the “x” cut line is shown atthe top of FIG. 1B. At this stage of the processing the cross sectionshows only the first material 20 of the fins, on top of the platform 10.The devices being processed usually are part of a larger IC on a chip.This is depicted on various figures by the wavy dashed line boundaries,indicating that the drawing typically may only be a small fraction of acircuit, or even a small fraction of only a single device structure.Various directions and dimensions are also indicated in FIG. 1B. Thewidth 21 the first material 20 of the fins is defined as the findimension substantially in parallel with the surface of the platform 10.The direction indicated by the arrow 22 is the one in which the heightof the fins is measured. The height direction is substantially verticalto the surface of the platform 10.

FIGS. 2A and 2B symbolically show respective inclusive, and crosssectional views after the epitaxial merging of the fins. The fins,typically both on the source side and the drain side, are merged in apairwise manner with an epitaxial second material 30. Epitaxialdeposition is a known technique of the VLSI fabrication art. Indescribing a structure, the adjective “epitaxial” is typically used toindicate that a particular material has been epitaxially deposited. Thestructural consequence of epitaxial deposition is that the depositedmaterial and the host material, at their common interface, have the samesymmetry and crystalline orientation. Further terms that may be used,such as “epitaxial relation”, “epitaxially”, “epitaxy”, “epi”,“epitaxial growth” etc. carry their customary usage, namely crystallinecontinuity across the interface. Typical techniques used in epitaxy mayinclude molecular beam epitaxy (MBE), chemical vapor deposition (CVD),ultra high vacuum CVD (UHCVD), rapid thermal CVD (RTCVD), or furtherknown methods.

The monocrystalline first material of the fins 20 may itself serve asthe seed surface for the epitaxial deposition of the second material 30.The epitaxy of the second material may have been carried out in aselective manner, meaning that the epitaxial growth occurs only overcertain surfaces, for instance over the fins, while other surfaces stayessentially free of the second material. Selective epitaxial growth hasknown techniques in the art. FIGS. 1A and 1B schematically depict thesituation where possible surplus epitaxial material has already beenremoved, for instance from the upper surfaces of the fins. Such removalmay take the form of various dry or wet etchings, including timed andselective etchings, or removal by polishing, as these are all knowntechniques in the art. In representative embodiments of the inventionthe second material 30 may have resulted from epitaxial growth on theside surfaces of the fins. A hard mask (not shown) on the upper surfacesof the fins may facilitate the selective growth on the sidewalls. Theepitaxial deposition may be stopped once the second material growing onthe sidewalls of neighboring fins fills up the space in-between thefins. At that point the fins are pairwise merged, a continuous materialhas been formed along the gate 60, and/or the sidewall 55. FIG. 2B showsthe cross sectional profile with the first material 20 of the finsalternating with the epitaxially deposited second material 30.

The second material 30 may have been so selected as to have differingetch properties in relation to the first material 20 of the fins. It maybe desirable that for certain selective etches the first material 20 ofthe fins etches at a faster rate than the second material 30. Suchrequirements may be satisfied in embodiments of the invention by severaldifferent combination of material selections. Some of the combinationsmy include the followings. The first material being essentially pure Si,and the second material being a SiGe alloy mixture with between 20% and60% Ge concentration. Or, the first material may essentially be pure Sior SiGe alloy mixture, and the second material may be Si:C with between1% and 5% C concentration. In alternate embodiments, the first materialmay be a SiGe alloy mixture, typically with less than 15% Geconcentration, and the second material 30 would also be a SiGe alloymixture, but with a differing Ge concentration than that of the firstmaterial. Typically, if the Ge concentration differs between the firstand second materials by at least 20%, that may be sufficient to lead tothe desired difference in etching rates. In alternate embodiments thefirst material may be a SiGe alloy mixture, typically with less than 15%Ge concentration, and the second material may be Si:C with between 1%and 5% C concentration. Or, first material may be a Si:C, and the secondmaterial being a SiGe alloy mixture with between 20% and 60% Geconcentration. In a further embodiments, the first material 20 may be aSiGe alloy mixture, with between 20% and 60% Ge concentration, and thesecond material 30 may be essentially pure Si. In further embodimentsthe second material may be a non-Si based semiconductor, for instance, acompound semiconductor, such as, without limiting intent, GaAs.

In representative embodiments of the present disclosure it may bedesirable to achieve low resistivity electrical connections between thefins, consequently one may dope the second material 30 to the same typeof conductivity as the first material 20 of the fins. Such doping may beperformed during the epitaxial growth of the second material, in socalled in-situ doping schemes, as known in the art. Both p-type andn-type dopants are well known in the art, and any of them may be used inthe embodiments of the present disclosure for doping both the firstmaterial 20 of the fins and second material 30.

A further consideration in selecting the first and second materials mayinvolve the state of stress in each. If the second material 30 has alarger lattice constant than the first material 20 of the fins, then attheir mutual interface the first material may be in a tensile state ofstress, and the second material in a compressive state of stress. It iswell known that the lattice constant of Si:C is smaller than that of Si,approximately in proportion with the carbon content. It is also knownthat the lattice constant of SiGe is larger than that of Si,approximately in proportion with the germanium content. Consequently,the selection of the second material 30 may also depend on the desiredstress state in the channels of the Fin-FET devices. As discussedalready, the channel is typically oppositely stressed than the source ordrain part of the fin. Consequently, for NFET devices, where the channelcarriers are electrons and tensile stress is advantageous in thechannel, one may select the first material 20 of the fins to beessentially Si or a SiGe alloy mixture with less than 15% Geconcentration, and the second material 30 to be Si:C with between 1% and5% C concentration. With such a selection the second material may imparta compressive stress onto the fins, resulting in a tensile stress in theFin-FET device channel. For PFET devices, where the channel carriers areholes and compressive stress is advantageous in the channel, one mayselect the first material 20 of the fins to be essentially Si, Si:C, ora SiGe alloy mixture with less than 15% Ge concentration, and the secondmaterial to be SiGe alloy mixture with between 20% and 60% Geconcentration, but typically with at least 20% higher Ge concentrationthan that of the first material. With such a selection the secondmaterial may impart a tensile stress onto the fins, resulting in acompressive stress in the Fin-FET device channel.

In further embodiments of the invention it may occur that the stress,which would arise from a lattice constant difference between the firstmaterial 20 of the fins and the second material 30, becomes relaxed dueto lattice defects, such as dislocations. In such embodiments thepotential mutual stress effects may be left out of consideration whenselecting compositions for the first and second materials.

FIGS. 3A and 3B symbolically show respective inclusive, and crosssectional views after the recessing of the fins. Following the epitaxialmerging of the fins, as schematically depicted in FIGS. 2A and 2B, aselective etch is applied to the Fin-FET devices. The selectivity of theetch is such that the first material 20 of the fins is removed at afaster rate than the second material 30. The degree of selectivity,namely the difference between the etch rates of the first and secondmaterials, is desired to be as high as possible. Details of the desiredfinal FET device's dimensions, and desired device performance maydetermine the minimum acceptable etching selectivity between the firstand second materials. In representative embodiments of the invention,one may aim to achieve a selectivity of at least 2 to 1, namely that thefirst material 20 of the fins etches twice as fast as the secondmaterial 30. Such selective etches are known in the art. For instance,Si can be etched by KOH, TMAH, or ammonia solutions selectively to SiGewith Ge content greater than 20%. Alternatively, one may implant the Sifirst with an n-type dopant, and then grow the Si:C or SiGe epitaxialsecond material. The doped silicon can be removed selectively withrespect to Si:C or SiGe using dry etch processes, known in the art foretching doped Si. If the first material is SiGe and the second materialessentially pure Si, one may use etching processes that selectively etchSi relative to SiGe. Such etches are know in the art, for instance, andwithout intent of limiting, H₂SO₄, hot ammonium hydroxide hydrogenperoxide and water (SCl), or dry etches that include HCl. In referenceto Si:C, Si etches at a higher rate in Secco etch, known in the art,than does Si:C.

FIGS. 3A and 3B schematically depict the FET device after the selectiveetching is completed. As shown, the first material 20 of the fins isrecessed in a vertically offset position relative to an upper surface ofthe second material 30. The recessing of the fins is instrumental inimproving stress coupling between the Fin-FET device channel and astress liner, which may be applied at a later stage of the fabrication.In an embodiment of the disclosure, following the selective etching, thegate sidewall 55, and possibly other layers around the gate 60, may bere-formed, or re-fabricated.

FIGS. 4A and 4B symbolically show respective inclusive, and crosssectional views after silicidation of the fins and of the mergingmaterial. Some embodiments of the invention aim for minimizing parasiticdevice resistance which may result from the recessing of the fins. Thefirst and second materials are of such kind that would allow forsilicide formation over both of them. Forming of silicide is a knowfabrication step in VLSI processing, and one may select for embodimentsof the instant application any of the known silicides of the art, forinstance, but not limited to Ni, Ti, Co, Pt silicides. The silicideformation may typically be carried out in a self aligned manner, asknown in the art. The silicide layer, as well, may impart stress ontothe fins and onto the merging material, which stress may be consideredwhen selecting the silicide material.

The common continuous layer of silicide 40 may lower the contactresistivity of the FET device, since the recess formation of the finsand the second material leads to an increase in the contact area. At thesame time, since the recess itself is in the fins, where proximitybetween a stress liner and the device channel is needed, channel stressmay increase.

FIG. 4A displays an embodiment where the mask layer 65 has been removedfrom the gate 60 prior to silicidation. During the silicidation of thefirst material and of the second material, a silicide layer 40′ is alsoformed over the exposed layer on the top of the gate, which layer may bea polysilicon layer. Silicide layer 40′ on top of the gate, may or maynot, be present in embodiments of the present invention.

FIG. 5 symbolically shows a cross sectional view after deposition of astress liner. The formation of the silicide 40 may be followed bydepositing a stress liner 50 over the FET device made up of the Fin-FETdevices. Deposition of a stress liner 50 may follow known processes ofthe art. One may typically select a nitride material for the stressliner. Depending on the details of the deposition process, the nitridematerial of the stress liner 50 may be implemented in either acompressive or in a tensile state of stress. For PFET devices one mayselect for stress liner 50 a nitride layer which is in a compressivestate of stress. Conversely, for NFET devices one may select for stressliner 50 a nitride layer which is in a tensile state of stress. Thestress liner may cover the whole of the FET, or only portions of it, forinstance, may only cover the silicided 40 portions.

In the foregoing specification, the invention has been described withreference to specific embodiments. However, one of ordinary skill in theart appreciates that various modifications and changes can be madewithout departing from the scope of the present invention as set forthin the claims below. Accordingly, the specification and figures are tobe regarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope ofpresent invention.

In addition, any specified material or any specified dimension of anystructure described herein is by way of example only. Furthermore, aswill be understood by those skilled in the art, the structures describedherein may be made or used in the same way regardless of their positionand orientation. Accordingly, it is to be understood that terms andphrases such as “under,” “upper”, “side,” “over”, “underneath” etc., asused herein refer to relative location and orientation of variousportions of the structures with respect to one another, and are notintended to suggest that any particular absolute orientation withrespect to external objects is necessary or required.

The foregoing specification also describes processing steps. It isunderstood that the sequence of such steps may vary in differentembodiments from the order that they were detailed in the foregoingspecification. Consequently, the ordering of processing steps in theclaims, unless specifically stated, for instance, by such adjectives as“before” or “after”, does not imply or necessitate a fixed order of stepsequence.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature, or element, of any or all the claims.

Many modifications and variations of the present invention are possiblein light of the above teachings, and could be apparent for those skilledin the art. The scope of the invention is defined by the appendedclaims.

1. A method for fabricating an FET device, the method comprising:receiving Fin-FET devices, wherein the Fin-FET devices have fins, andthe fins are composed of a first material; merging the fins by anepitaxial deposition of a second material; recessing the fins verticallywith a selective etch, wherein the selective etch removes the firstmaterial at a faster rate than the second material; and forming in aself aligned manner a continuous silicide layer over the first materialand over the second material.
 2. The method of claim 1, wherein themethod further comprises: depositing a stress liner overlaying the selfaligned silicide layer.
 3. The method of claim 2, wherein the methodfurther comprises: selecting the stress liner to be a nitride layer. 4.The method of claim 1, wherein the method further comprises: in-situdoping the second material during the epitaxial deposition.
 5. Themethod of claim 1, wherein the method further comprises: selecting thesecond material in such manner that after the epitaxial deposition thesecond material imparts a stress onto the fins.
 6. The method of claim1, wherein the method further comprises: selecting the first material tobe essentially Si, SiGe alloy mixture, or Si:C, and selecting the secondmaterial to be a SiGe alloy mixture with between 20% and 60% Geconcentration.
 7. The method of claim 1, wherein the method furthercomprises: selecting the first material to be essentially Si or SiGealloy mixture, and selecting the second material to be Si:C with between1% and 5% C concentration.
 8. The method of claim 2, wherein the methodfurther comprises: selecting the FET device to be an NFET device, andselecting the stress liner to be in a tensile state of stress.
 9. Themethod of claim 2, wherein the method further comprises: selecting theFET device to be a PFET device, and selecting the stress liner to be ina compressive state of stress.
 10. An FET device, comprising: aplurality of Fin-FET devices, wherein the Fin-FET devices comprise fins,and the fins are composed of a first material; a second material,wherein the second material is epitaxially merging the fins to oneanother in pairwise manner, and wherein the fins are recessed in avertically offset position relative to an upper surface of the secondmaterial; and a continuous silicide layer formed over the fins and overthe second material.
 11. The FET device of claim 10, further comprising:a stress liner overlaying the continuous silicide layer.
 12. The FETdevice of claim 11, wherein the stress liner is a nitride layer.
 13. TheFET device of claim 10, wherein the first and the second material areboth doped and are of a same type of conductivity.
 14. The FET device ofclaim 10, wherein the second material imparts a state of stress onto thefins across an epitaxial interface between the first material and thesecond material.
 15. The FET device of claim 10, wherein the firstmaterial is essentially Si, SiGe alloy mixture, or Si:C, and the secondmaterial is a SiGe alloy mixture with between 20% and 60% Geconcentration.
 16. The FET device of claim 10, wherein the firstmaterial is essentially Si or SiGe alloy mixture, and the secondmaterial is Si:C with between 1% and 5% C concentration.
 17. The FETdevice of claim 11, wherein the FET device is an NFET device, and thestress liner is in a tensile state of stress.
 18. The FET device ofclaim 11, wherein the FET device is a PFET device, and the stress lineris in a compressive state of stress.